Global positioning system (GPS) based automated guidance systems for maneuvering work vehicles traveling along prescribed paths have found applications in precision farming operations to enhance productivity and reduce farming input cost. A GPS based work vehicle guidance system with a real-time kinematics (RTK) base station has achieved position measurement accuracy of one inch in terms of the position of GPS antenna mounted on the roof of the vehicle cab. A GPS based work vehicle guidance system with differential error correction signal from commercial satellites has achieved position measurement accuracy of four inches in terms of the position of GPS antenna mounted on the roof of the vehicle cab. The measured vehicle position is used by a GPS guidance algorithm to calculate a vehicle deviation from a prescribed path. The calculated deviation is further used by the GPS guidance algorithm to calculate a desired steering action, i.e., a steering command in terms of steering angle or steering rate, to correct the deviation and direct the vehicle to the prescribed path. During an automated vehicle guidance operation, the steering command is calculated periodically at the same update rate as that of GPS signal, generating a time sequence of steering commands in real time. Thereafter, a steering control system executes the steering commands, by actuation of the vehicle steering mechanism to the desired steering action in order for the vehicle to track the prescribed path.
A vision based automated guidance system uses visual images in a forward-looking view field to identify vehicle deviation from a desired path that is usually marked by row crops. The identified deviation is then used by a vision guidance algorithm to calculate a steering command, i.e., a desired steering action, to correct the deviation and direct the vehicle to the desired path. During an automated vehicle guidance operation, the steering command is calculated periodically at the same update rate as that of image processing, generating a time sequence of steering commands in real time. Thereafter, a steering control system executes the steering commands, by actuation of the vehicle steering mechanism to the desired steering action in order for the vehicle to track the prescribed path.
Known steering control techniques use conventional PID control which is a combination of proportional, integral and derivative control. The term PID is widely used because there are commercially available modules that allow for the user to set the values of each of the three control types. The PID control law is able satisfactorily to meet the specifications for a large portion of control problems, and the user simply has to determine the best values of the three control types. However, because PID control is based on linear system theory, it may not satisfactorily handle the full operating range of systems with severe nonlinearity and uncertainty. As a steering mechanism for a work vehicle presents severe nonlinearity problems and uncertainty problems, a conventional PID steering controller can not deliver robust performance.
Uncertainties in the deadband and the gain value of the steering mechanism may result from variations during the manufacturing process, variations in hydraulic pump supply pressure, variations of the ground resistance, and the like. As a result, a conventional PID steering control system requires field calibrations of control parameters, such as varying deadband values and gain values so that the control system performs well with particular components of the steering mechanism and condition of a field. This calibration is typically required upon replacement of components of the steering mechanism, and re-calibration may periodically be required to compensate for changes in the steering mechanism due to wear and the like. This calibration requirement is time consuming and usually frustrating for a non-technically oriented operator. Because PID steering control determines steering performance during normal steering operation, a robust controller with good performance is desirable.
Conventional PID steering controllers are designed to achieve zero error and continue to make steering corrections back and forth in opposite directions around zero error even when the steering error is very small. For example, due to nonlinearities in an electro-hydraulic steering mechanism, such as the deadband nonlinearity of a steering valve, these steering corrections result in persistent directional switching of the steering valve and considerable back and forth movement of a steering valve spool even for very small steering corrections. This zero-error control effort can also result in persistent steering cylinder rod push-pull vibrations around its regular course of movement. This means that a conventional PID steering controller results in unnecessary accelerated wear of the steering valve and steering cylinders.
In conventional PID steering control systems, dynamic performance degrades due to saturation nonlinearities of a steering mechanism when a steering error is large. The resulting response is typically slow with large over shoot and under shoot as well as long settling time. Improved responsiveness in the presence of large steering errors is desirable.
Therefore what is sought is a control system and method that overcomes one or more of the problems or shortcomings set forth above.